I strongly agree with the sentiment to use what makes the crushed grain look right. There is variability in barley, and you can set your mill differently if you're crushing pils malt, wheat or black patent, for instance. Also, if you spray your malt with water to do the malt conditioning trick I wrote about in the Sierra Nevada Beer Camp article for Zymurgy, you'll be able to set your mill tighter.
I use an old Listerman Phil Mill I with a drill and a converted 5 gallon plastic water jug as a grain hopper. There is no way I could figure out the gap setting without dismantling it. On other mills, do you have a direct measurement or are you relying on the settings on some knob? Any time you are using an indirect measurement, you have to worry about it being properly calibrated.
Learn what a proper crush looks like, and set your mill accordingly. Check it every time. With heavy use, and all the vibration from the drill, the settings can change over time. Run some through, catch it in your hand, and visually inspect it.
That said, if you know your system and know what setting gives you that result, obviously start there since it will save you time. But still verify the actual crush.